Image forming apparatuses such as digital copy machines, laser printers, or facsimile machines are widely used. Such an image forming apparatus uses an optical scanning unit that scans a laser beam. When an image is formed in the image forming apparatus, after a photoreceptor is charged by a charging unit, an electrostatic latent image is formed on the photoreceptor by the optical scanning unit by writing according to image information. The electrostatic latent image on the photoreceptor is developed by toner supplied from a development unit. The toner image developed on the photoreceptor is transferred to recording paper by a transfer unit and is fixed on the recording paper by a fixing unit to obtain a desired image.
Along with speeding up of color image forming apparatuses such as digital copy machines and laser printers, a tandem-mode apparatus is in practical use which includes a plurality of photoreceptors in tandem arrangement. In this case, for example, four photoreceptor drums are arranged in the conveying direction of the recording paper; each photoreceptor is simultaneously exposed to light by a scanning optical system corresponding to each photoreceptor drum to form a latent image; and these latent images are developed by development units that use developers with different colors such as yellow, magenta, cyan, and black. The developed toner images are sequentially transferred on the same recording paper in an overlapping manner to obtain a color image.
As compared to a mode that forms each color image with one photoreceptor sequentially, since the tandem mode exposing a plurality of photoreceptors simultaneously can output color and monochrome images at the same speed, the tandem mode is advantageous in high-speed printing. On the other hand, since scanning optical systems corresponding to a plurality of photoreceptors are necessary, a unit for exposing the photoreceptors tends to be large in size and the challenge is to miniaturize the unit. Another challenge is to eliminate color drift when the toner image developed on each photoreceptor is transferred to the recording paper in an overlapping manner.
With regard to the tandem-mode image forming apparatus as described above, for example, Japanese Laid-Open Patent Publication Nos. 2004-109700 and 2004-109699 disclose an optical scanning unit that includes a wedge-shaped prism disposed on an optical path from a light source unit to deflecting means and a writing start position correcting means for rotating and adjusting the wedge-shaped prism around an optical axis to vary a position of a beam spot in the sub-scanning direction, and the optical scanning unit can control the beam spot position on a photoreceptor drum during the writing of image data. At the time of continuous printing, relative color drift of each color can be corrected effectively to output a good color image with less color drift.
Japanese Laid-Open Patent Publication No. 2004-233638 discloses a lens adjustment apparatus that includes: a light source; a device that divides the light emitted from the light source into four light beams; an adjustment device that drives a second lens on a plane where a normal line is the optical axis of the second lens; a diffraction grating that diffracts the collected light beams from the first and second lenses to generate interference; a micromotion stage that drives the diffraction grating in the direction including a component of a direction vertical to the groove direction of the grating plane; four interference image observation systems that are constituted by an objective lens, an imaging lens, and a CCD camera to observe the interference light; a processing device that processes the four interference images to detect one aberration from aberrations sensitive to the decentering of the second lens; and a control device that detects an adjustment amount from the detected aberration to drive the adjustment device, and the lens adjustment apparatus can adjust a lens with small NA highly accurately.
Japanese Laid-Open Patent Publication No. 2002-90675 discloses an optical scanning unit that includes: an optical deflection device that deflects light in a predetermined direction; a plurality of laser elements; a pre-deflection optical system that includes a glass lens and a plastic lens to convert a cross-sectional shape of light emitted from each laser element into a predetermined shape; and a post-deflection optical system that includes two lenses forming an image such that each of the light deflected by the optical deflection device is scanned on a predetermined image plane at a constant speed. The power of the two lenses of the deflection optical systems is regulated to be positive relative to the direction orthogonal to the rotation axis of the reflection face of the optical deflection device. At least one of the lens surfaces of each of the lenses is formed to be a lens without rotational symmetric surfaces. In this way, an optical scanning unit can be provided which is suitable for an image forming apparatus that can provide a color image without color drift at low cost.
In the optical scanning unit, a multiplicity of optical elements such as lenses and mirrors is disposed at optimum positions. Such optical elements are disposed to guide an optical path of a light beam emitted from a light emitting element composed of a laser diode, to adjust the beam shape, and to illuminate a photoreceptor accurately. In some cases, such optical elements must be adjusted in positions or angles at the time of the assembly adjustment of the unit or after the subsequent operation.
For example, if a mirror is displaced which is disposed before a lens making the light beam converge on the photoreceptor, for example, the writing position of the light beam on the photoreceptor is displaced from an appropriate position, which makes a problem that the quality of the image formed on the photoreceptor is deteriorated. The same applies to other optical elements constituting the optical scanning unit.
With regard to the adjustment of the optical elements, it is preferred that highly accurate adjustment can be easily performed. In this case, adjusting means of the optical element must be able to adjust the angle and position thereof highly accurately without affecting the characteristic of the optical element.